In recent years, there is an increasing need for the use of electric waves in response to a ubiquitous network society, and a wireless personal area network (WPAN) which realizes wireless broadband at home or a millimeter waveband wireless system, such as a millimeter-wave radar, which supports safe and secure driving starts to be used. An effort to realize a wireless system at a frequency equal to or greater than 100 GHz is actively made.
In regard to second harmonic evaluation of a wireless system in a 60 to 70 GHz band or evaluation of a radio signal in a frequency band equal to or greater than 100 GHz, as the frequency becomes high, the noise level of a measurement device and conversion loss of a mixer increase and frequency precision is lowered. For this reason, a high-sensitivity and high-precision measurement technology of a radio signal over 100 GHz has not been established. In the conventional measurement technology, it is not possible to separate harmonics of local oscillation from the measurement result, and there is difficulty in strict measurement of unnecessary emission or the like.
In order to overcome the problems in the related art and to realize high-sensitivity and high-precision measurement of a radio signal in a frequency band equal to or greater than 100 GHz, it is necessary to develop a narrowband filter technology of a millimeter waveband for the purpose of suppressing an image response and a high-order harmonic response, and in particular, there is a demand for a technology which is adaptable to a variable frequency type (tunable).
In order to realize this, the applicant has suggested a millimeter waveband filter in which a Fabry-Perot resonator used in an optical field is applied to millimeter waves and desired frequency components of the millimeter waves are selectively transmitted by a resonance action between a pair of electric wave half mirrors arranged to face each other inside a transmission line allowing propagation in a TE10 mode (single mode) (Patent Document 1).